1. Field of the Invention
The present invention relates to an optimum control for internal combustion engines, which feedback-control the ignition timing and air-fuel ratio so as to improve the output and fuel consumption rate of an engine.
2. Description of the Prior Art
The ignition timing for most internal combustion engines is adjusted, unless there exists any special reason such as the requirements from knocking or exhaust gas characteristics, in accordance with the operating conditions of the engine, namely, on the basis of an engine speed, intake pressure, etc., so as to simultaneously attain a maximum output and a minimum fuel consumption rate for the engine. However, the effect of these prior art methods is limited and they inevitably suffer some losses in both engine output and fuel consumption rate. For instance, such losses are caused by such factors as the variations in performance among manufactured engines, the variations in ignition timing correction, the changes in ambient conditions, etc.
Techniques have been devised for feedback-controlling the ignition timing of an engine so as to eliminate such losses and provide a maximum engine output. In an example of these ignition timing feedback control techniques, the ignition timing of an engine is controlled at an optimum ignition timing which provides a maximum engine torque by selecting at least two ignition timings which are different from each other and are in the vicinity of but apart by a given ignition angle from a calculated ignition timing obtained in accordance with the operating conditions of the engine, operating the engine at the at least two selected ignition timings alternately for a given time period, detecting a signal indicative of the engine speed when the engine is operated at each of the at least two ignition timings, comparing with one another the engine speed signals obtained at at least three successive operating points during the operation of the engine at each of the at least two selected ignition timings, deciding whether the calculated ignition timing is advanced or retarded from an optimum ignition timing (the minimum spark advance for best torque or MBT) and then correcting the calculated ignition timing in accordance with the result of the decision.
In accordance with this technique in which a change in the output is determined on the basis of a change in the engine speed, although the engine speed varies naturally due to various factors, it is possible to discriminate whether the actual change in the engine speed is due to the variation of the ignition timing or any other external factor (e.g., an accelerator pedal operation) and to prevent erroneous correction control of the ignition timing in a direction opposite to the optimum ignition timing by interrupting the ignition timing correction during acceleration and deceleration periods, slope ascending and descending periods, etc.
With the above-mentioned prior art control technique, however, if the response of the engine speed is slow when the engine is operated for a given time period at each of the two different ignition timings, since the engine speeds are obtained in terms of average values during the respective given time periods and they are compared with each other, the ability of detecting the difference of the changes of the engine speed becomes insufficient and thus the length of the given time period has to be extended to make up for the insufficient ability.
In the description to follow, the operation of intentionally varying a control variable for controlling the operation of the engine and the resulting change in the operating condition to thereby effect the optimum control of the engine is referred to as "dither" and the period of time during which the dither is effected is referred to as a "dither period". Also, an amount of change of the control variable for effecting the dither is referred to as a "dither quantity".
Further, an internal combustion engine is hereinafter simply referred to as an engine.
The present invention has been made with a view to solving the foregoing problems in the prior art.